The present disclosure relates to X-ray imaging and, more particularly, to a dynamic X-ray tube output function combined with a high-ratio, high primary transmission grid system for use in mammography.
Transmission X-ray imaging involves a point source (sometimes referred to as a focus or X-ray focus) of X-rays and a collimator to limit the X-rays to the region of interest. When the X-rays pass through the object, X-ray attenuation differences due to structures in the object give rise to differences in transmitted X-ray intensity. These intensity differences are in turn detected by an image receptor giving rise to the detected X-ray image. The detected X-ray image is comprised of two parts. The primary image consists of detected X-rays that have traveled on a straight-line path from the source to the image receptor. The secondary image consists of detected X-rays that have interacted with atoms and electrons in the object and were deflected or scattered from their original path (scattered X-rays). These scattered X-rays form a diffuse, out-of-focus image that is superimposed on the primary image. X-ray image contrast is reduced by scattered X-rays with the problem becoming more acute as the thickness and density of the object being imaged increases. In mammography the structures of clinical importance have very little radiographic contrast, so it is important to reduce the contribution of scattered radiation to the detected X-ray image to a minimum.
In 1915 anti-scatter grids were introduced to improve image contrast in general radiography (G. Bucky, Method and Apparatus for Projecting Roentgen images, U.S. Pat. No. 1,164,987). Somewhat later, anti-scatter grids were introduced to improve image contrast in mammography (D. Richter, 20th Annual Meeting of the AAPM, San Francisco, Jul. 30, 1978). As illustrated in FIG. 1, a conventional mammographic anti-scatter grid consists of an array of radiopaque foil strips or septa, interspersed with strips of radiolucent interspace material. The septa are composed of material that absorbs X-rays, such as, but not limited to lead, while the interspace material is composed of material that does not substantially absorb X-rays, such as, but not limited to, polycarbonate fibers or paper. The width of the individual septa are indicated in FIG. 1 as xe2x80x9cd,xe2x80x9d while the width of the interspace material is indicated as xe2x80x9cD.xe2x80x9d By combining the width of the septa (xe2x80x9cdxe2x80x9d) and the interspace material (xe2x80x9cDxe2x80x9d), the grid pitch, or grid period, can be determined (xe2x80x9cd+Dxe2x80x9d).
The grid is positioned in the X-ray beam after the object so that the X-rays traveling in a straight-line path from the focus through the breast to the image receptor (primary X-rays) strike only the edges of the radiopaque septa. The septa are thus projected onto the image receptor as lines. To address this issue, during an exposure the grid is commonly moved orthogonal to the lines through a distance of at least 20 grid pitches to blur out the X-ray shadows of the lines. When these shadows are not eliminated, the result is an image flaw known as a xe2x80x9cgridline artifact.xe2x80x9d On the other hand, scattered X-rays do not travel in a straight line from the focus to the image receptor, but are deflected within the breast and approach the grid at an angle, and have a much greater probability of striking the sides of the septa and being absorbed. As a result, the contribution of scattered X-rays to the detected X-ray image is reduced and the image contrast is improved. In mammography, the grids employed typically absorb 30% to 40% of the primary X-rays and 75% to 85% of the scattered X-rays.
A variety of techniques have been suggested for suppressing gridline artifacts. The simplest is to use a high line density grid, in which the number of septa per centimeter is sufficiently high that the image receptor is incapable of recording the image of the grid lines. Alternately, the grid can be moved during the exposure. The motion of the grid has the effect of blurring the grid line artifacts, reducing their visibility on the final image. Typically, the grid needs to be moved by more than 20 times the grid pitch in order to adequately suppress the grid line artifacts; however, given a constant tube output during the exposure the artifacts can be completely suppressed by moving the grid any integral number of grid pitches.
One problem with moving the grid is that the available distance is limited, and it is possible during long exposures for the grid to run out of travel space. The direction of motion can then be reversed, but this raises the possibility of a gridline artifact to develop while the grid is stationary. One technique that has been developed and is in clinical use is to use a variable velocity. The grid is moved quickly during the start of the exposure, guaranteeing that even for short exposures the grid moves at least 20 times the grid pitch. Then the grid is progressively slowed, to ensure that even for long exposures the grid does not reach the end of its travel space.
In 1931 Potter suggested a technique he described as xe2x80x9cfeatheringxe2x80x9d (H. E. Potter, Am. J. Roent., Vol XXV (May 1931), pp. 677-683). In this approach the tube output is gradually increased at the beginning of an exposure and gradually decreased at the end of the exposure. This has the effect of blurring the edges of the grid line artifacts. However, Potter did not describe how to accomplish the feathering, and did not implement this approach on a clinical x-ray machine.
In mammography, the anti-scatter grid is typically integrated into a removable device called a Bucky Housing. The X-rays pass successively through
a) the breast,
b) the breast support (the upper surface of the Bucky housing),
c) the anti-scatter grid,
d) the image receptor, and
e) the automatic exposure control (AEC) sensor.
The AEC sensor is used to measure the amount of radiation (X-rays) that has reached the image receptor. This allows the X-ray generator to terminate the exposure when the desired exposure level has been reached.
The efficiency of an anti-scatter grid is reduced if it is not properly aligned. For a properly made grid, the primary planes of the septa all intersect along a line known as the focal axis (FIG. 2A). The distance from the focal axis to the grid is the focal length of the grid. When the grid 5 is properly aligned (FIG. 2A), the focal spot 4 lies on the focal axis 6, and any x-rays traveling directly from the focal spot (primary x-rays) either pass between the septa or strike the septa edge and are absorbed. In this orientation, the projections of the septa on the image receptor are minimized. If the grid 5 is not aligned (FIG. 2B), then the focal spot 4 does not lie on the focal axis 6, a fraction of the primary x-rays strike the sides of the septa, and a higher fraction of the primary X-rays are attenuated than desired. This condition can be caused by poor initial positioning, but can also occur when the grid is moved to blur the septa shadows, when there are manufacturing defects in the grid, or when the distance from the focus to the grid changes from its ideal value.
Grid ratio is an important parameter in determining the effectiveness of an anti-scatter grid. Grid ratio is defined as the ratio of the height of the radiopaque septa (indicated as xe2x80x9chxe2x80x9d in FIG. 1) to the interspace distance between septa (the width of the interspace material indicated as xe2x80x9cDxe2x80x9d in FIG. 1). The higher the grid ratio, the more efficient the grid is in controlling scattered X-rays. However, as the grid ratio increases, grid alignment difficulties are compounded. Also, if the grid ratio of a conventional grid is increased by increasing the height of the septa (xe2x80x9chxe2x80x9d), attenuation of the primary X-rays by the interspace material increases. If the grid ratio is increased by decreasing the interspace distance (xe2x80x9cDxe2x80x9d) without a corresponding decrease in the thickness of the individual septa (xe2x80x9cdxe2x80x9d), attenuation of the primary X-rays by the septa increases. Typically, in mammography the grid ratio ranges from 4:1 to 6:1, and the strip densities range from 25 to 50 septa per centimeter. Previous attempts at using conventional higher ratio grids have met with limited success because the advantage of increased scatter control has been largely offset by the increased attenuation of the primary X-rays and by alignment difficulties.
The realization of these limitations of conventional anti-scatter grids in mammography has led to the development of alternative anti-scatter techniques. The scanning multiple slit assembly is representative of these techniques and virtually eliminates scatter at the expense of limitations in the ability to position the breast being imaged. In this method, multiple pre-patient slits and post-patient articulated slots are scanned across the image receptor (as described in U.S. Pat. No. 4,096,391). Images acquired through this technique display significantly more contrast than images acquired under the same conditions with a conventional grid (M. V. Yester, G. T. Barnes, and M. A. King, Med Phys 8:155-162, 1981; G. T. Barnes, X. Wu and A. G. Wagner, Medical progress Through Technology 19:7-12, 1993).
Another approach to improve X-ray image quality in mammography has been to employ a high ratio, air interspace grid. The use of air as the interspace material allowed for grids with increased septa height (xe2x80x9chxe2x80x9d) (and therefore a higher grid ratio) without an increase in the primary X-ray attenuation by the interspace material. These grids were curved, with a radius of curvature equal to the distance to the X-ray focus. Such an approach allowed the grid to remain focused throughout the exposure, virtually eliminating scatter without increasing primary attenuation (R. J. Jennings, T. R. Fewell, and J. Vucich, Radiology 181(P):234, 1991) (J. D. Robinson et al, Radiology 188:868-871, 1993). To ensure adequate rigidity of the grid, it was necessary to make the septa fairly thick (high value of xe2x80x9cdxe2x80x9d). Consequently, to ensure low primary attenuation it was necessary to use a coarse grid spacing (high value of xe2x80x9cDxe2x80x9d). However, in order to eliminate gridline artifacts these grids needed to be scanned through a distance of more than twenty times the grid pitch (the sum of xe2x80x9cDxe2x80x9d+xe2x80x9cdxe2x80x9d). Because of the coarse septum spacing, the need to scan through twenty or more grid pitches, and the curved grid, these grids required a housing so bulky to make the use of certain commonly used patient positions impossible in mammography. These considerations prevented clinical acceptance of the system.
The linear grids thus described preferentially absorb scattered X-rays that travel across the septa, and preferentially transmit scattered X-rays that travel along the septa. U.S. Pat. No. 1,164,987 taught a cellular anti-scatter grid. This comprises two sets of linear septa that intersect each other at right angles. The X-rays that are preferentially transmitted by one set of septa are preferentially absorbed by the other set. Consequentially, the transmission of scattered X-rays through a cellular grid is lower than for a linear grid of similar grid ratio.
U.S. Pat. No. 5,606,589 teaches a practical implementation of the cellular grid. A plurality of thin copper foil sheets are photo etched to create a square array of air holes and septa. The etched sheets are aligned and bonded to form laminated two-dimensional focused grid panels. These grid panels typically have a grid ratio of 2.4-3.5:1. Applications of a two-dimensional photo-etched grid have been limited by high manufacturing costs and relatively low grid ratio. The grid ratio is limited both by the difficulty in manufacturing and by the problem of grid alignment.
Important to the present disclosure is control of the tube current of the X-ray tube. The tube current in a given tube is determined primarily by the filament temperature and to a lesser degree by the X-ray tube voltage. The filament temperature is in turn determined by the filament current; however, the filament temperature changes slowly in response to changes in filament current, with characteristic times of tens to hundreds of milliseconds. Before 1970, X-ray generator designs utilized open-loop control of the tube current. The filament current necessary to achieve a desired tube current was set during calibration. Just prior to an exposure the filament current was turned on and it was maintained throughout the exposure.
More recent X-ray generator designs utilize closed-loop control of the tube current (W. T. Sobol, Med. Phys. 29 (2), February 2002, pp. 132-144). This is accomplished by incorporating an electronic circuit that measures the tube current and compares it to the desired tube current. The filament current is then automatically adjusted to achieve the desired filament temperature and tube current.
In view of image contrast limitations and the prior art, there exists a need for spatially compact, high-ratio, high primary transmission anti-scatter grid systems. A particular need exists in the field of mammography, where high image contrast is difficult to obtain with conventional systems, but critically required for early diagnosis of cancerous tissues.